1. Field of the Invention
The present invention relates to an apparatus and method for optimally controlling damping force characteristics of four road wheel suspension units (hereinafter referred to as shock absorbers) of an automotive vehicle.
2. Description of Background Art
A Japanese Patent Application First Publication No. Heisei 4-27612 published on Jan. 30, 1992 exemplifies a first previously proposed damping force characteristic control apparatus.
In the first previously proposed damping force characteristic control apparatus, a plurality of vertical sprung mass acceleration sensors are disposed on a vehicle body located adjacent to the respective shock absorbers and they detect the vertical sprung mass accelerations at the respective road wheel positions. Vertical sprung mass velocities are calculated from the vertical acceleration signals of the respective vertical acceleration sensors, and a plurality of relative displacement sensors are disposed between a sprung mass and an unsprung mass so as to detect the relative displacements between the road wheels (unsprung mass) and vehicle body (sprung mass) so that vertical relative velocities at the respective road wheel positions are calculated.
When a product between each corresponding one of the vertical sprung mass velocity signals and that of the relative velocity signals is equal to or above a predetermined value, the damping force characteristic of the corresponding one of the shock absorbers is controlled using a hard damping force characteristic. On the contrary, when the product is below the predetermined value, the damping force characteristic of the corresponding one of the shock absorbers is controlled using a soft damping force characteristic.
A Japanese Patent Application First Publication No. Heisei 4-191111 published on Jul. 9, 1992 exemplifies a second previously proposed damping force characteristic control apparatus for respective shock absorbers interposed between a corresponding vehicular body (sprung mass) and tire wheel assemblies (unsprung mass).
In the above-identified Japanese Patent Application Publication (Heisei 4-191111, a plurality of actuators (shock absorbers) are each interposed between the vehicle body and a rear tire (road) wheel so as to enable an increase or decrease in a suspension force (damping force characteristic) on the vehicle body with respect to the corresponding rear tire (road) wheel. A vertical acceleration sensor is arranged so as to detect a vertical acceleration acted upon the vehicle body due to a vibration input from front tire wheels caused by a recess and convex formed on a road surface on which the vehicle is running. A vehicle speed sensor is arranged so as to detect the vehicle speed, and a control unit is arranged so as to control operations of the actuators on the basis of input signals from the above-described sensors that are provided in the vehicular suspension control apparatus.
The control unit described above is constructed so as to actuate the actuators, when determining that the vehicular body vertical acceleration detected from each of the vertical acceleration sensors exceeds a predetermined value, calculates a delay duration in time for which the rear road wheels have reached to the corresponding recess and convex portion of the road surface which has given the sprung mass vertical acceleration which exceeds the predetermined value and actuates the actuators described above according to the magnitude of the control signal derived by inverting a signal based on the vertical sprung mass acceleration after the calculated delay duration in time has passed upon passage of the front tire (road) wheels on the recess and convex portion of the road surface.
That is to say, in the above-identified Japanese Patent Application First Publication No. Heisei 4-191111, even when a relatively large vibration occurs on the vehicle body at the time when the front road tire wheels have passed such a recess and convex portion of the road surface as described above, the control for the rear road wheel side shock absorbers is carried out so as to cancel the vertical acceleration acted upon the vehicle body by referring to the vertical acceleration acted upon the vehicle body derived at the time when the front road wheels have passed on the recess and convex portion of the road surface, when the rear road wheels pass on the same recess and convex portion of the road surface. Consequently, when the rear road wheels have passed on the same recess and convex portion on the road surface, the vibration input derived at the time when the front road wheels have passed the same recess and convex portion of the road surface can be reduced.
It is noted that, in the above-described second previously proposed vehicular shock absorber control apparatus disclosed in the Japanese Patent Application First Publication No. Heisei 4-191111, a preview control such that a timing at which the control signal is used to control the damping forces of the shock absorbers located at the rear road wheel positions is retarded according to a vehicle speed is carried out and no elimination of the phase deviation of the filter processed signal is carried out.